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Investigation of Shear and Fracture Properties of FRP using Digital Image Correlation

For the determination of the interlaminar and intralaminar shear strength, strain and modulus of laminate composites, more than a dozen measurement techniques exist, several of which are supported by ASTM, ISO or DIN standards. One reason for so many attempts is the difficulty in obtaining a reasonably pure and uniform shear stress state in the test specimen.

Typically, linear strain gauges and rosette strain gauges have been used to measure deformations for the determination of the in-plane (intralaminar) elastic modulus and the strain to failure. The advent of optical strain measurement techniques such as digital image correlation (DIC), provide new opportunities to generate high resolution maps of the shear strain field as a function of the globally applied strain. DIC allows for the easy full-field mapping of the deformations (in-plane and out-of-plane for stereo DIC) and strains (normal and shear), revealing the pattern of deformation and damage throughout the specimen. In this work, it is shown that non-contact measurements of the strain field are an important enhancement to measuring the shear properties of FRP composites.

In-plane testing of V-notched specimens and 10° off-axis coupons will be compared in terms of intralaminar shear moduli and shear strengths. Additionally, the interlaminar properties determined via 3pt, 4pt and 5pt flexural testing (requiring small span-length to thickness ratios for the shear loading to become dominant) will be compared. The latter test methods are limited to a determination of the apparent interlaminar strength, while the full stress-strain behavior cannot be determined. The double beam shear test allows for a calculation of the shear modulus, provided that four additional elastic, in-plane properties have been determined in advance. The novelty includes the use of the DIC determined shear strains which are used within the elastic loading range to evaluate the shear modulus.

Besides material properties such as modulus and strength, also fracture toughness values are needed as input data for cohesive zone models for computational material science purposes. The current work shows the use of DIC for fracture testing of FRP under Mode I and Mode II (including a comparison of 2 types of tests) loading conditions, revealing the critical strains as well as crack tip opening displacements in the vicinity of the crack tip as useful data for numerical models.

Dr.-Ing. Matthias J. Merzkirch
National Institute of Standards and Technology